The present invention relates to a method and system for controlling operation of a rotary disk refiner that processes fiber. In particular, the invention relates to a method and system of regulating operation of a rotary disk refiner in response to a refiner process variable preferably in response to a set point.
Many products we use every day are made from fibers. Examples of just a few of these products include paper, personal hygiene products, diapers, plates, containers, and packaging. Making products from wood fibers, cloth fibers and the like, involves breaking solid matter into fibrous matter. This also involves processing the fibrous matter into individual fibers that become fibrillated or frayed so they more tightly mesh with each other to form a finished fiber product that is desirably strong, tough, and resilient.
In fiber product manufacturing, refiners are devices used to process the fibrous matter, such as wood chips, pulp, fabric, and the like, into fibers and to further fibrillate existing fibers. The fibrous matter is transported in a liquid stock slurry to each refiner using a feed screw driven by a motor. Each refiner has at least one pair of circular ridged refiner discs that face each other. During refining, fibrous matter in the stock to be refined is introduced into a gap between the discs that usually is quite small. Relative rotation between the discs during operation causes the fibrous matter to be fibrillated as the stock passes radially outwardly between the discs.
One example of a refiner that is a disc refiner is shown and disclosed in U.S. Pat. No. 5,425,508. However, many different kinds of refiners are in use today. For example, there are counterrotating refiners, double disc or twin refiners, and conical disc refiners. Conical disc refiners are often referred to in the industry as CD refiners.
Each refiner has at least one motor coupled to a rotor carrying at least one of the refiner discs. During operation, the load on this motor can vary greatly over time depending on many parameters. For example, as the mass flow rate of the stock slurry being introduced into a refiner increases, the load on the motor increases. It is also known that the load on the motor will decrease as the flow rate of dilution water is increased.
During refiner operation, a great deal of heat is produced in the refining zone between each pair of opposed refiner discs. The refining zone typically gets so hot that steam is produced, which significantly reduces the amount of liquid in the refining zone. This reduction of liquid in the refining zone leads to increased friction between opposed refiner discs, which increases the load on the motor of the refiner. When it becomes necessary to decrease this friction, water is added to the refiner. The water that is added is typically referred to as dilution water.
One problem that has yet to be adequately solved is how to control refiner operation so that the finished fiber product has certain desired characteristics that do not vary greatly over time. For example, paper producers have found it very difficult to consistently control refiner operation from one hour to the next so that a batch of paper produced has consistent quality. As a result, it is not unusual for some paper produced to be scrapped and reprocessed or sold cheaply as job lot. Either way, these variations in quality are undesirable and costly.
Another related problem is how to control refiner operation to repeatedly obtain certain desired finished fiber product characteristics in different batches run at different times, such as different batches run on different days. This problem is not trivial as it is very desirable for paper producers be able to produce different batches of paper having nearly the same characteristics, such as tear strength, tensile strength, brightness, opacity and the like.
In the past, control systems and methods have been employed that attempt to automatically control refiner operation to solve at least some of these problems. One common control system used in paper mills and fiber processing plants throughout the world is a Distributed Control System (DCS). A DCS communicates with each refiner in the mill or fiber processing plant and often communicates with other fiber product processing equipment. A DCS monitors operation of each refiner in a particular fiber product processing plant by monitoring refiner parameters that typically include the main motor power, the dilution water flow rate, the hydraulic load, the feed screw speed, the refiner case pressure, the inlet pressure, and the refiner gap. In addition to monitoring refiner operation, the DCS also automatically controls refiner operation by attempting to hold the load of the motor of each refiner at a particular setpoint. In fact, many refiners have their own motor load setpoint. When the motor load of a particular refiner rises above its setpoint, the DCS adds more dilution water to the refiner to decrease friction. When the motor load decreases below the setpoint, dilution water is reduced or stopped.
During refiner operation, pulp quality and the load on the refiner motor vary, sometimes quite dramatically, over time. Although the aforementioned DCS control method attempts to account for these variations and prevent the aforementioned problems from occurring, its control method assumes that the mass flow of fibrous matter in the stock entering the refiner is constant because the speed of the feed screw supplying the stock is constant. Unfortunately, as a result, there are times when controlling the dilution water flow rate does not decrease or increase motor load in the desired manner. This disparity leads to changes in refining intensity and pulp quality because the specific energy inputted into refining the fibrous matter is not constant. These changes are undesirable because they ultimately lead to the aforementioned problems, as well as other problems.
Hence, while some refiner process control methods have proven beneficial in the past, they in no way have resulted in the type of control over finished fiber product parameters and the repeatability of these parameters that is desired. Thus, additional improvements in refiner process control are needed.
A system for and method of monitoring and controlling operation of a disc refiner. The method regulates operation of a refiner in response to a refiner process variable preferably in relation to a setpoint. In one preferred implementation, the process variable is based on a temperature. In another implementation, the process variable is based on a pressure. In still another preferred implementation, the process variable is based on a stock consistency. In a further preferred implementation, operation of the refiner can be regulated in response to a refiner energy parameter or a parameter related thereto.
In one implementation, the volumetric flow rate of stock entering the refiner is regulated. In another implementation, the flow rate of dilution water entering the refiner is regulated. In still another implementation, both the stock volumetric flow rate and the dilution water flow rate are regulated.
In one preferred implementation, the volumetric flow rate of stock is regulated in response to a measured or calculated refiner temperature. In another preferred implementation, the dilution water to the refiner is regulated based on the refiner temperature.
In one preferred implementation, the volumetric flow rate of stock is regulated in response to a measured or calculated refiner pressure. In another preferred implementation, the dilution water to the refiner is regulated based on the refiner pressure.
In another preferred implementation, the dilution water to the refiner is regulated based on stock consistency. In still another preferred method, the volumetric flow rate of the stock is regulated based on stock consistency.
If desired, two or more of these parameters can be regulated based on the same process variable. For example, regulation of volumetric flow rate and dilution water can both be based on refiner temperature. Regulation of volumetric flow rate and dilution water can also both be based on refiner pressure. If desired, regulation of volumetric flow rate and dilution water can also both be based on stock consistency.
The refiner temperature is a temperature of stock inside the refiner or adjacent its inlet or outlet. In one preferred implementation, the refiner temperature is a temperature of stock in the refining zone. Where there is more than one sensor in the refining zone, the temperature can be provided by a particular selected sensor or calculated based on the sensor data from more than one sensor. In one preferred embodiment, temperature measurements from multiple sensors are averaged.
The refiner pressure preferably is a pressure of stock inside the refiner, such as a pressure in the refining zone, or a pressure inside the refiner adjacent the refiner inlet or outlet. Where there is more than one sensor in the refining zone, the pressure can be provided by a particular selected sensor or calculated based on the sensor data from more than one sensor. In one preferred embodiment, pressure measurements from multiple sensors are averaged.
Stock consistency can be determined using a consistency sensor upstream or downstream of the refiner. Where a consistency sensor is used, the sensor is located upstream of the refiner, preferably adjacent the refiner inlet.
Stock consistency can also be determined using a novel method that is based on a temperature or a pressure (or both) inside the refiner, preferably inside the refining zone. In one preferred implementation, the method uses temperature or pressure measured inside the refining zone along with other refiner parameters in determining the consistency of stock in the refining zone as a function of time and location in the refining zone. This method advantageously permits consistency of stock to be determined in real time in the refining zone.
A refiner energy related parameter includes refiner energy or power measured in real time. Other refiner energy related parameters include motor load, refiner gap, refiner plate force, hydraulic energy input, or another refiner energy related parameter.
Where volumetric stock flow is regulated, it preferably is regulated by controlling the speed of a feed screw that provides the refiner with stock. Where dilution water flow is regulated, it preferably is regulated by controlling operation of the dilution pump. Other refiner parameters can be controlling using the method of this invention.
So that the process can be controlled despite changes in refiner operation not due to regulation using the method, one preferred implementation pauses to permit refiner operation to stabilize before resuming regulation of refiner operation. For example, where an operator manually changes refiner operation, regulation is paused preferably until refiner operation stabilizes. The same is true where a refiner is also subject to control of a processing device, such as a Distributed Control System (DCS).
In one preferred embodiment, the method is implemented in the form of a controller that preferably is a PI or a PID controller. If desired, a proportional controller can be used. The controller can be a digital or analog controller and can be configured to operate with a digital processor such as a personal computer, a DCS, a programmable controller or the like.
The system includes a processor that receives data related to refiner operation. Suitable data includes data related to the process variable or variables used in regulating refiner operation. In one preferred embodiment, the processor receives data related to one or more of the following parameters: the power inputted into the refiner, the feed screw speed (or volumetric stock flow or feed rate), the temperature of the stock before it enters the refiner, the temperature of stock after it leaves the refiner, a refiner temperature, a refiner pressure, the force exerted on the refiner disks urging them together, the dilution motor power of the dilution pump, the chip washing water temperature, the dilution water temperature, the gap between the refiner disks, as well as other parameters.
In carrying out the method, the processor outputs at least one control signal. Each control signal can be directly provided to the refiner or a component related to the refiner, such as the feed screw or dilution water pump. If desired, each control signal can be provided to another processor, such as a DCS, that causes the DCS to regulate the desired parameter. For example, a control signal can be provided to the DCS that causes the DCS to change feed screw speed. Another control signal can be provided to the DCS that causes the dilution water flow rate to change.
One preferred embodiment of the system uses one or more sensors in the refining zone to provide sensor data from which a process variable calculation or measurement can be made. In one preferred embodiment, the one or more sensors are temperature sensors but can be pressure sensors or a combination of temperature and pressure sensors.
In one preferred embodiment, each sensor is carried by a refiner disk or segment of the disk. In one preferred sensor disk or sensor disk segment, each sensor is imbedded in the refining surface of the disk or segment.
In a preferred sensor embodiment, the sensor has a sensing element carried by a spacer that spaces the sensing element from the material of the disk or segment in which it is imbedded. One preferred spacer is made from an insulating material that preferably thermally insulates the sensing element from the thermal mass of the refiner disk material.
Other objects, features, and advantages of the present invention include: a monitoring and control system and method that is simple, flexible, reliable, and robust, and which is of economical manufacture and is easy to assemble, install, and use.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating at least one preferred embodiment of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.